As far as most paper, board and paper board grades are concerned, there is a need to reduce the rate of liquid penetration into the structure of the paper, board, or paper board through the addition of hydrophobic substances during the paper manufacturing process. Copying paper, writing and printing papers, papers for so called inkjet printers, and paper board for packages for juice, milk and other liquids are examples of such paper, board, and paper board grades.
These grades for their proper use need to have some liquid repellent features. Different methods are available or the achievement of that effect. One of them includes the addition of an emulsion for dispersion of a hydrophobic material during the paper manufacturing process.
Aqueous emulsions and dispersions of hydrophobic cellulose-reactive type are previously known per se. These emulsions and dispersions, however, are manufactured of only a single hydrophobic, cellulose-reactive sizing agent.
Many different hydrophobic materials can be used. The so called hydrophobic, cellulose-reactive sizing agents belong to the most efficient ones. It is believed that the hydrophobic effect, when using that type of agent, is achieved through a reaction between the hydrophobic material and the hydroxyl groups of the cellulose.
The hydrophobic, cellulose-reactive materials which are most commonly used in connection with neutral or weakly alkalic conditions during the paper or paper board manufacturing process are alkyl ketene dimer (in the following called AKD) or alkenyl succinic acid anhydride (in the following called ASA).
Alkyl ketene dimer, which are used in the first place when the demands on a durable and permanent hydrophobation are great, can react with the cellulose when the paper or the paper board is being dried wherein beta-ketoesters are established. In the figure below, the reaction between AKD and cellulose and the hydrolyse reaction are illustrated: ##STR1##
The reaction is slow at low pH values and in practice AKD can not be used other than in the neutral or weakly alkali pH range. Instead of reacting with the fibre during the paper or paper board manufacturing, AKD also can be hydrolysed such that an unstable beta-keto acid is obtained, which can be decarbonated to corresponding ketone. The hydrolysed products which are formed during the paper manufacturing process, i.e. the ketone products that are obtained, are very disturbing for certain types of photo copying apparatuses, in which these ketones may form depositions.
The degree of the hydrolysis reaction for AKD, however, is substantially smaller in comparison with ASA.
This implies that AKD usually is supplied to the paper or board mill in the form of a dispersion or emulsion which has a particle size of between 0.2 to 2 micrometers. The dispersion or emulsion usually is stabilised by means of a cation-active polymer. This polymer, which also gives the particles their charge, usually consists of cation-active starch but also use of anion-active or amphoteric starches or synthetic polymers occur.
AKD has a melting point of between 15-60.degree. C., depending on the length of the fatty tail. The commercial products which are most commonly used so far often have a mixture of C.sub.14, C.sub.16, and C.sub.18 hydrocarbons, implying a melting point of between 40-60.degree. C.
AKD is particularly sensitive in the presence of extractive agents of the fatty acid type, because such agents are saponificated at alkalic pH values and also because they compete with the distribution on the cellulose surface. A poor AKD distribution will result in a poor chemical reaction with the fibre. Groundwood, thermo-mechanical pulps and chemi-mechanical pulps therefor are difficult to hydrophobate by means of AKD.
The most pronounced drawback with AKD, however, is that the hydophobicity is developed slowly. A completely developed sizing is not achieved until after several days after the manufacturing in the paper or the board machine.
Another drawback is that a too large dosage of AKD causes friction problems in connection with the paper or the board/paper board. AKD also impairs the toner addition in connection with the photocopying processes as well as the adherence of the plastic layer in connection with the manufacturing of plastic laminated materials.
Alkenyl succinic acid anhydride (ASA) is another example of a so called cellulose-reactive hydrophobation agent. The anhydride can react with the hydroxyl groups of the cellulose, wherein a transesterification takes place. In comparison with AKD, ASA has a much higher reactivity with the cellulose and hence also with water, wherein hydrolysis of the product is a greater problem for ASA than for AKD.
ASA is delivered to the paper mill in the form of oil, sometimes with emulgator added. Emulsification takes place in a special emulsification equipment in connection with the paper or board manufacturing machine (so called in situ production) and cationic, anionic or amphoteric starch or other polymer is used as a stabiliser and charge carrier. The particle size of the emulsion usually is 1-5 micrometers. Larger particles are hydrolysed more slowly but smaller particles provides a better distribution on the cellulose fibres, wherefore there exists an optimal particle size. ASA thus is hydrolysed quickly and the hydrolysis product and its salts with Ca.sup.2+ are sticky and frequently give rise to depositions in the paper or paper board manufacturing process. The hydrolysis as well as the reaction velocity are increased with increased pH, but the reaction is fast over the entire pH range. The hydrophobicity is developed fast and is usually fully developed is the finishing part of the paper and paper board manufacturing process.
Below, the reaction between ASA and cellulose is illustrated: ##STR2##
where R or R' is an alkenyl.
In connection with the more and more common use of precipitated calcium carbonate, so called PCC, as a filler during the paper and paper board manufacturing, the need of a hydrophobic sizing agent is increased considerably for the achievement of the same degree of sizing as is achieved in a corresponding process where e.g. chalk is used as a filler. Moreover, often problem occur which have to do with the fact that the hydrophobation effect is reduced after a period of time.
To sum up, one can thus state that both the two hydrophobation agents have specific advantages and drawbacks when used in the paper or paper board manufacturing process as well as in the finished paper or paper board product.